The present invention relates generally to a new technique for manufacturing an integrated circuit nickel silicide/salicide structure, and more particularly to a structure and method that do not result in nickel silicide “remnants” which cause the bridging or “pipe formation” that may result in internal electrical short-circuiting. (The term “salicide” refers to self-aligned silicide.)
It is well known that the electrical connection which is achievable by forming aluminum conductors directly on a doped electrode (e.g., a doped electrode of a source, drain, or polycrystalline gate layer) of an integrated circuit transistor has higher contact resistance than is desirable. Aluminum-to-doped-electrode connections also have the additional problem of junction spiking due to aluminum migration during subsequent high temperature processing. Today's state-of-the-art electrode contact structures involve using a silicide/salicide followed by a barrier metal and subsequently deposited conductor material consisting of aluminum or copper, depending on the technology, as indicated in the publication “SILICON PROCESSING FOR THE VLSI ERA”, Vol. 2, Process Integration, S. Wolf, Lattice Press.
In the past, titanium silicide contacts have been commonly used in processes having device geometry dimensions greater than approximately 0.25 microns, and cobalt silicide contacts are used for processes having device geometry dimensions in the range from about 130 nanometers to 250 nanometers. For more recent integrated circuit manufacturing processes having device geometry dimensions of 130 nanometers or less, nickel silicide contacts are now being used instead of cobalt silicide contacts because the manufacture of nickel silicide contacts is much more compatible with the very narrow polycrystalline silicon gate electrode conductors of the newest processes than is the case for cobalt silicide contacts. This probably is because grains in nickel silicide are substantially smaller than the grains in cobalt silicide and therefore are more capable of forming silicide over the extremely narrow polycrystalline silicon gate conductors without silicide bridging or pipe formation between polycrystalline silicon gate conductors and source or drain electrodes of CMOS transistors. Also, nickel silicide contacts do not display certain undesirable leakage or Schottky characteristics that often are present in cobalt silicide contacts.
FIG. 1 shows a typical N-channel CMOS transistor structure after the N+ source region 2 and the N+ drain region 3 have been formed in a lightly doped P-type silicon layer 1. A very thin (e.g., less than 25 Angstroms) gate dielectric layer 4 has been formed on a surface area of P-type layer 1 between the edges of source region 2 and drain region 3, as shown. A doped polycrystalline silicon gate layer 5 has been formed on gate dielectric layer 4. An oxide layer 7 with the indicated openings also has been formed, as shown.
FIG. 2 shows the structure of FIG. 1 after oxide side-wall “spacers” 6A and 6B have been formed on the surface areas of P-type layer 1 along the opposed edges of polycrystalline silicon gate conductor 5, as illustrated. Spacers 6A and 6B typically are formed of silicon nitride, silicon oxide, or a combination of silicon nitride and silicon oxide. Spacers 6A and 6B are provided to prevent the subsequently described gate-to-source and/or gate-to-drain “nickel silicide bridging” or “nickel silicide pipe formation” from occurring between polycrystalline silicon gate 5 and source region 2 and/or drain region 3 of the illustrated transistor.
FIG. 3 illustrates the structure of FIG. 2 after a nickel layer 10 has been deposited on its upper surface using conventional nickel deposition techniques. A titanium nitride “cap” layer 12 has been deposited on nickel layer 10. (Titanium and cobalt also have been utilized as a metal layer 10 to form titanium silicide or cobalt silicide. Titanium nitride has been used as a cap layer for protecting the “siliciding” metal layer from oxidation.) Cap layers such as cap layer 12 are used to prevent oxidation of the nickel metal from which the nickel silicide is formed and also to provide thermal stability during subsequent annealing.
The next step in the conventional procedure of nickel silicide formation is to subject the structure shown in FIG. 3 to a first annealing step at a temperature of approximately 250-300 degrees Centigrade for approximately 30 seconds. Then a standard etch back process is performed. The standard etch back process has been performed using ammonium hydroxide/peroxide (SCl)), followed by a sulfuric peroxide mixture (SPM).
Referring to FIG. 4, the above-mentioned first annealing results in formation of di-nickel silicide (Ni2Si) layer 14A at the surface of source region 2, Ni2Si layer 14B at the surface of drain region 2, and Ni2Si layer 16 at the upper surface of polycrystalline silicon layer 5, as shown. Some nickel metal remains on the upper surfaces of side-wall spacers 6A and/or 6B.
FIG. 5 shows the structure that results after cap layer 12 and un-reacted nickel of layer 10 have been etched away. At this point, all of the un-reacted nickel metal theoretically should have been stripped off and there should be no nickel silicide left on the side-wall spacers 6A and 6B. The desired nickel silicide layers 14 and 16 should be ready for the next annealing step, and for an interconnect metallization step after that.
But as a practical matter, some nickel remnants often remain on the surfaces of side-walls 6A and 6B. In FIG. 5, reference numeral 8 illustrates an example of undesired nickel silicide bridging or nickel silicide pipe formation that may cause electrical shorting and reduction of manufacturing yield.
The above described nickel silicide bridging or pipe formations have been a substantial problem and have caused substantially reduced manufacturing yields.
The structure of FIG. 5 ordinarily includes a second phase of nickel silicide formation. The second phase ordinarily is accomplished by means of a second annealing procedure at a temperature in the range of approximately 400-550 degrees Centigrade for approximately 30 to 60 seconds in order to form the desired nickel silicide (NiSi) phase.
Thus, there is an unmet need for a way of manufacturing integrated circuits including nickel silicide electrode contact layers so as to avoid nickel silicide bridging or pipe formation that results in internal electrical short-circuits.
There is another unmet need for a way of manufacturing integrated circuits in which nickel silicide remnants that lead to bridging or pipe formation are avoided or eliminated.
There is another unmet need for a way of manufacturing integrated circuits in which the etchant chemistry completely eliminates any un-reacted nickel that may cause nickel silicide bridging or pipe formation.